drm/i915: Split off pci_driver.remove() tail to drm_driver.release()

In order to support driver hot unbind, some cleanup operations, now
performed on PCI driver remove, must be called later, after all device
file descriptors are closed.

Split out those operations from the tail of pci_driver.remove()
callback and put them into drm_driver.release() which is called as soon
as all references to the driver are put.  As a result, those cleanups
will be now run on last drm_dev_put(), either still called from
pci_driver.remove() if all device file descriptors are already closed,
or on last drm_release() file operation.

Signed-off-by: Janusz Krzysztofik <janusz.krzysztofik@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190530133105.30467-1-janusz.krzysztofik@linux.intel.com